As of 2015, the K1 fiance visa requires a number of documents including the application, a valid passport, police certificates, medical exams, proof of financial support, payment of fees, proof of relationship with the fiance and photographs, notes State.gov. These are the requirements of the person coming to the United States, which are in addition to the documents that the U.S. citizen must file.
U.S. citizens requesting permission for their fiance to enter the country must provide several things. The requirements include proof that the requester is a citizen, proof that there is an intention to marry within 90 days, proof that both parties are free to marry and proof of meeting in person at least once in two years. The last requirement can be waived in two situations: if meeting would violate social or cultural customs or if meeting is an extreme hardship on the requester, states USCIS.gov.
If the fiance coming into the country has children, they need a K2 visa. This can be applied for at the same time as the K1. The U.S. citizen needs to list the children on the paperwork with the fiance's information. A K2 application needs to be filed and all fees paid individually for each child. A child must be unmarried, and to be able to have a stepparent status established, the parents' marriage must occur before the child is 18 years of age.